The present invention relates to an apparatus for controlling an inverter operated in a parallel mode, with respect to another power supply, and in synchronism with a reference frequency signal of a stand-by commercial power supply.
In order to improve the system's reliability, a parallel operating system of plural inverters is often employed in a uninterruptible power supply system. Another type of power supply system, which is also widely employed for the same purpose, includes a changeover switch connected between the output of inverters operating in parallel and a stand-by commercial power supply (a reserved power source). In this power supply system, if the parallel operating inverters are subjected to a system breakdown, or when the inverters are to be thoroughly checked for maintenance and repair, the changeover switch is switched so that a load can be uninterruptedly and continuously supplied with power from the stand-by commerical power supply.
In such a system, it must be ensured that the output phases of the respective inverters match with each other, and also that the output phase of the inverters matches that of the stand-by commercial power supply. According to a prior art, the above phase-matching control is performed by an analog technique.
However, owing to an operating point offset, electrical characteristic fluctuation, operating point temperature drift, aging etc., of operational amplifiers and other associated circuits, an analog control apparatus requires complex circuitry and/or precise adjustments for compensating for these shortcomings. Further, to ensure the stable operating of the apparatus over a long period of time, it is necessary to inspect and readjust the apparatus within a relatively short cyclic period of time, in order that the aging of the control apparatus can be properly compensated for.